1. Field of the Invention
The present invention relates to fuel vapor processing apparatus for processing fuel vapor (for example, gasoline vapor) produced within fuel tanks.
2. Description of the Related Art
Vehicles represented by automobiles have a canister as a fuel vapor processing apparatus for processing gasoline vapor (vaporized gasoline) produced within a fuel tank. Generally, the canister has a case formed as a hollow container and an adsorption material disposed within the case. The case has a tank port for drawing gasoline vapor produced within the fuel tank and an atmospheric port for drawing atmospheric air. The adsorption material consists of activated carbon granules that adsorb or desorb the gasoline vapor. More specifically, the gasoline vapor is temporally adsorbed by the adsorption material in the canister and desorbed from the adsorption material under predetermined conditions.
When the adsorption material adsorbs the gasoline vapor, so-called adsorption heat is generated because this process is an exothermic process. On the other hand, when the gasoline vapor is desorbed from the adsorption material, the adsorption material is cooled because this process is an endothermic process to increase in a kinetic energy.
As the adsorption material adsorbs gasoline vapor, the temperature of the adsorption material increases. This in turn reduces the adsorption ability of the adsorption material. As the gasoline vapor is desorbed from the adsorption material, the adsorption material is cooled. This in turn reduces the vapor desorption ability of the adsorption material.
Mixing a heat storage material with an adsorption material is a well-known technique for preventing such degradation of the adsorption and desorption abilities of the adsorption material. This technique of mixing the heat storage material can maintain the adsorption and desorption abilities of the adsorption material during the exothermic process and the endothermic process.
Because the amount or volume of the adsorption material to be contained within the canister is previously determined, the size of the canister depends on the amount or volume of the heat storage material to be mixed with the adsorption material. Therefore, it is necessary to reduce the amount of the heat storage material in order to minimize the size of the canister. Japanese Laid-Open Patent Publication No. 2005-282481 sets out a technique for minimizing the amount or volume of the heat storage material by changing the mixing amount of the heat storage material.
However, there still exists a problem in the above well-known technique. Although a part of the adsorption material filled adjacent to a surrounding wall constituting the case and separating inside and outside of the case can easily exchange heat with the outside via the surrounding wall, another part of the adsorption material filled distant from the surrounding wall is difficult to exchange heat with the outside.
More specifically, it is difficult for the adsorption material positioned in a central area of the case to exchange heat with the outside. Therefore, heat and chill tend to remain in a part of the adsorption material filled in the central area of the case compared to the other parts of the adsorption material positioned in other areas of the case. Accordingly, the adsorption and desorption abilities of the adsorption material have become less effective.
Therefore, there is a need in the art for a fuel vapor processing apparatus that can improve a fuel vapor adsorption performance.